Try these steps...
* Open your calculator and remove the logic board
* Inspect the logic board, especially around the connectors and look for corrosion.
* Inspect both sides of the board.
* Put your board in a container with >90% isopropyl alcohol and let it sit for a while.
* Use a soft brush, like a toothbrush and lightly brush away any corrosion you see.
* Rinse in alcohol and repeat.
* Let it air dry for a day.
* Re-assemble and hope for the best.